Prostitution is illegal in Korea and yet their sex industry accounts for 1.6% of their GDP ($13 billion) according to The Korea Women's Development Institute in 2007. Serious issues of human trafficking, and organized crime come to mind as you glance down these alleyways.
These "glass houses" are a common sight around large bus and train stations and urban legend has it that the clientelle are Koreans only--foreigners are excluded.
As sad as it is sex still sells... even in Korea.